Radio communication systems and networks were originally developed primarily to provide voice services over CS networks. The CS networks only provide voice services as are considered to be legacy networks. PS networks may comprise or represent a communication network that groups all transmitted data into suitable sized data blocks called packets. Examples of specific PS networks that may be used in certain embodiments of the described network include, but are not limited to, legacy PS networks such as the second generation (2G), 2.5 generation (2.5G) and third generation (3G) networks, and/or evolved packet switched (EPS) networks, and/or all internet protocol (IP) based PS networks.
Legacy PS networks, for example, the so-called 2.5G and 3G networks, have enabled network operators to provide data services as well as voice services. However, network architectures are now evolving toward IP networks, which provide both voice and data services over a PS network. However, network operators have a substantial investment in existing CS and legacy PS network infrastructures and would, therefore, typically prefer a gradual migration to the IP network architectures. This will allow them to extract sufficient value from their current investments.
Migrating from existing CS and legacy PS networks to an all IP network will require a substantial investment in new network infrastructure to include the capabilities needed to support the next generation radio communication applications. Network operators can minimise this investment by deploying hybrid networks by re-using legacy network infrastructure and overlaying the next generation radio communication system and applications over it. For example, a next generation network could be overlaid onto an existing CS or legacy PS network in the transition to an all IP-based network. This allows networks to evolve from one generation to the next while providing backwards compatibility for legacy equipment.
The evolution to IP networks is evident, for example, the so-called Universal Mobile Telephone System (UMTS), is a legacy PS network commonly known as a 3G radio communication system, but has evolved into using enhanced PS network technologies such as High Speed Packet Access (HSPA) technology. In addition, air interface technologies within the UMTS framework have begun to evolve towards new air interface technologies defined in the so-called Long Term Evolution (LTE) and LTE-Advanced systems. Target performance goals for LTE systems include, for example, support for 200 active calls per 5 MHz cell and sub 5 ms latency for small IP packets.
The next generation radio communication systems and networks such as LTE and LTE-Advanced are considered to be all IP networks. These networks will have an upgraded PS network infrastructure called the evolved packet system (EPS). The EPS includes an evolved packet core (EPC) that forms the basis of the core PS network for the all IP network. These enhanced PS networks will provide all the mobile core functionality that, in the previous generations (2G, 2.5G, and 3G), has been realised through the existing CS networks and legacy PS networks. Each new generation, or partial generation, of a radio communication system and network needs further complexity and capabilities from existing systems, the demands of which are expected to increase with future enhancements to current systems before they are completely replaced.
As enhanced PS LTE systems are introduced, it is anticipated that they will need to interact with, for example, legacy CS and PS communication systems such as the 2/2.5G Global System for Mobile Communications (GSM) radio communication systems and the 3G UMTS radio communication systems, respectively. The 3rd Generation Partnership Project (3GPP) Technical Specification (TS) 23.292 is one of the 3GPP standards that describes the architectural requirements for delivery of consistent services to a user regardless of the attached access type such as access to CS networks or access to any of the current and future PS networks.
The IP Multimedia Subsystem (IMS) architecture or IMS was developed in order to, among other things, ease the migration from existing CS and PS based networks to the all IP networks. Based on the 3GPP standards, the IMS will serve the user as a single service engine in the new PS networks. These standards also describe IMS Centralized Services (ICS) where a user's services are migrated from a CS network to an IMS based network such as an all IP network based on LTE/LTE Advanced. This means that the IMS will have to handle all originating and terminating calls.
When a calling party (user A) places a call to a called party (user B) the call set-up process involves an originating call associated with user A and a terminating call associated with user B. The terms “originating call” and “terminating call” may comprise or represent the connection set-up signalling in relation to user A or user B between user equipment, respectively. Examples of originating or terminating calls that may be used in certain embodiments of the described network, include but are not limited to, the connection set-up signalling enabling a communication connection to be made between user A and user B in the two call halves model. The originating call is the connection set-up signalling for user A in the first call half and the terminating call is the connection set-up signalling for connecting the call with user B in the second call half.
The user equipment (UE) may comprise or represent any device used for communications. Examples of user equipment that may be used in certain embodiments of the described network are wireless devices such as mobile phones, terminals, smart phones, portable computing devices such as lap tops, handheld devices, tablets, netbooks, computers, personal digital assistants and other wireless communication devices, or wired communication devices such as telephones, computing devices such as desktop computers, set-top boxes, and other fixed communication devices.
FIGS. 1a and 1b illustrate a network 100 highlighting the evolution from existing CS networks, legacy PS networks, towards an IMS based IP network. FIG. 1a illustrates a generalized schematic of an evolved network 100 having an enhanced mobile switching center server (MSC-S) 116 in the CS network that is configured to communicate directly with nodes of the IMS 108. FIG. 1b illustrates a generalized schematic of a network 140 in which legacy nodes of the CS network like a non-enhanced mobile switching center (MSC) 134 communicates indirectly with the IMS 108 via other specialized IMS network nodes (detailed below).
Referring to FIG. 1a, the evolved network 100 illustrates various nodes associated with a serving Public Land Mobile Network (PLMN) 101 and a home PLMN 102. The serving PLMN includes a CS network 104, a PS network 105 including a legacy PS network 106 and an EPS network 107. The CS network 104 includes UE 114 in communication with some registrar nodes associated with the various access domains (or networks) such as the MSC-S 116 and Media Gateway (MGW) 118. The MSC-S 116 has IMS Centralized Services capabilities, meaning that it has the capability to register users or UEs such as UE 114 directly in the IMS 108. Since the MSC-S 116 and MGW 118 have IMS functionality, they can also be considered part of the IMS 108.
The term subscriber server may comprise or represent a user database that includes subscription-related information or subscriber profiles to assist call handling within a network or group of networks such as the CS, PS networks and/or IMS. Examples of subscriber servers that may be used in certain embodiments of the described networks are home subscriber servers (HSS) or home location registers (HLRs) that act as subscriber servers for existing CS networks, legacy PS networks, EPS networks, all IP networks, and/or the IMS. The 3GPP HSS includes functionality for acting as a subscriber server in the CS networks, legacy PS networks, EPS networks and IMS and performs authentication and authorization of the UE, and can provide information about the subscriber's location and IP information.
The home PLMN 102 includes some IMS architecture 108, a 3GPP HSS node 110 and a remote end 112 of the connection. The home PLMN 102 includes further registrar nodes in the IMS 108 associated with the various access domains (or networks) i.e. a call session control function (CSCF) 120 in the home PLMN 102. The CSCF 120 can include a proxy CSCF (P-CSCF) 120a, serving CSCF (S-CSCF) 120b and/or interrogating CSCF (I-CSCF) 120c (not shown). As can be seen, only a few of the relevant IMS nodes are shown in FIG. 1a. These include the CSCF 120, a Service Centralization and Continuity Application Server (SCC AS) 122a, which provides, among other functions, a Terminating-Access Domain Selection (T-ADS) function 122b. In FIG. 1a, the legacy PS network 106 includes a Serving GPRS Support Node (SGSN) 126, and the EPS network includes a Mobility Management Entity (MME) 124, both of which can communicate with the 3GPP HSS node 110 during, for example, the T-ADS function 122b. 
In this example, UE 114 is an originating UE having access to the CS network 104 and is in communication via MSC-S 116 for setting up a call with another UE (not shown) in the remote end 112 of the connection. As the UE 114 is in the CS network 104, a terminating call (also known as a CS terminating call) associated with the UE in the remote end 112 is routed through MSC-S 116 to the IMS 108 via the CSCF 120. Omitted from the figure, among other things, are the access point nodes, e.g., eNodeBs. On receipt of the terminating call the IMS 108 triggers the SCC-AS 122 a to perform the T-ADS function 122 b for deciding the type of network, PS or CS network access, the terminating call can be routed to.
The T-ADS function 122b operates to, among other things, obtain access information related to:                (a) being aware of the currently used access type for a particular connection, i.e., PS or CS network access (for forwarding terminating calls to users and/or UEs),        (b) checking for IMS voice over packet switched (VoPS) support and RAT (Radio Access Type) type in the serving (MME 124 and/or SGSN 126, and        (c) querying, for all terminating calls for registered contacts (if registered via a PS network), the current serving nodes (via a subscriber server such as 3GPP HSS 110) for IMS VoPS support and RAT type.        
The T-ADS function 122b obtains the aforementioned access information (VoPS support & RAT) via the reference point Sh. The 3GPP HSS 110 obtains this information via the reference points S6a from the EPS network 107 via MME 124 and/or from the legacy PS network 106 via Gr/S6d to SGSN 126.
The procedures performed by the T-ADS function 122b are further described in the 3GPP standards documents 3GPP TS 23.292, 23.221, 23.401. Conventionally, the procedures of the T-ADS function 122b are triggered by the SCC-AS 122a in IMS 108 based on an Sh query. This is the conventional T-ADS PS Support and RAT information retrieval when the network includes an MSC-S 116.
Referring to FIG. 1b, the network 140 is shown with a set of network nodes associated with a serving PLMN 130 and home PLMN 132 in which the CS network 104 has some legacy nodes. The same reference numerals used in FIG. 1a are reused in FIG. 1b identifying the same or similar network nodes. FIG. 1b illustrates schematically a conventional T-ADS PS Support and RAT information retrieval for a non-enhanced or a conventional (legacy) mobile switching centre (MSC) 134 in network 140. In FIG. 1b, the CS network 104 includes the MSC 134, which does not have the capability to register users or UEs in the IMS 108. Accordingly, and as an example, a terminating call from a call originating from UE 114 will be routed through MSC 134 and on to a gateway MSC (GMSC) 136. The GMSC 136 then routes the terminating call through protocol conversion entities media gateway control function (MGCF) 138 and MGW 118 in the control and media planes.
The 3GPP TS 23.221 standard sets out the steps for deciding the serving domain or network (e.g. CS network or PS network) for an originating and a terminating call. This is referred to as service domain selection (SDS). For example, the 3GPP standard 23.221 outlines which serving network, e.g. CS, PS network, or IMS, should serve a call arriving at a terminating GMSC (e.g. GMSC 136) in the CS network 104.
The 3GPP TS 23.292 standard describes the T-ADS function and requires that all terminating calls must first be handled by the IMS. This assumes that there is sufficient hardware/software present in the network to handle the required capacity demand resulting from handling all terminating calls in the IMS.
One scenario in which the CS/PS domain/network selection is foreseen to be an issue is when ICS is introduced in alignment with the start of voice over LTE (VoLTE). Multimedia Telephony Service (MMTel)/IMS will be the recommended service engine. This means that during early phases of IMS and VoLTE deployments, both VoLTE PS network access and CS network access will co-exist due to a lack of full LTE coverage. The users or UEs in such a network will be served by the CS network and the LTE based PS network, with the IMS being used as the service engine. For these scenarios, the call signaling related to calls originated in CS access and terminating calls are required to be routed to or handled by the IMS. This must occur even when the terminating user or UE, which is the called party, associated with the terminating call is on CS network access. All terminating calls will need to visit or be handled by the IMS to execute terminating services for the user (or subscriber) associated with the terminating call.
If the majority of the calls in a network are between users having CS network access, which will initially be the case in the migration to an all-IP based network, then the routing of or handling of all originating and terminating calls in the IMS will add severe connection delays until sufficient IMS hardware/software capacity has been deployed throughout the network. This means a substantial initial investment, which is not desirable. Therefore, there is a significant need to optimise the handling of originating and/or terminating calls in the network during the migration to minimise the connection delays caused by insufficient hardware/software capacity in the IMS.